The present invention relates to a machine for mounting (positioning and then fixing) flexible printing plates on plate-holder cylinders of flexographic printing machines and for printing proofs with the cylinders thus prepared.
Machines such as the one described in Italian patent no. 953251 in the name of the same Applicant are available for mounting printing plates and printing proofs and are generally known as combined optical proofing and plate mounting devices; hereinafter, for the sake of brevity, they will be referenced to as optical plate mounting devices.
Said known optical plate mounting devices are provided, in a front region, with a counterpressure drum that is installed horizontally and rotatably between the side walls of the machine and with a vertically sliding underlying beam provided with semicircular supports for supporting in succession a plate-holder cylinder so that it can rotate parallel to the drum.
The counterpressure drum is covered with a sheet of paper on which registration lines (or other markings) are applied: these registration lines run along straight generatrices and circular directrices of the drum and are usually marked by means of an appropriate writing stylus with which the plate mounting device is provided.
Double-adhesive sheet material is used to fix one or more printing plates on a plate-holder cylinder (or on a jacket wrapping around it) when said cylinder, supported by said beam, is in the so-called mounting position, which is at a certain distance from the counterpressure drum.
In order to precisely position the printing plates on a plate-holder cylinder, and therefore to fix the printing plates on said cylinder, the operator usually uses an appropriate optical apparatus provided with a semitransparent mirror; this apparatus is usually located frontally in current plate mounting devices, is arranged parallel to the counterpressure drum, has essentially the same length as the drum, and is movably mounted between an active position, which is lowered and intermediate between the counterpressure drum and a plate-holder cylinder in the mounting position, and a spaced position, which corresponds to proof printing.
By means of this apparatus, the operator aligns directly-viewed points of the printing plate located on the plate-holder cylinder, with reflected points of said sheet of paper wrapped around the counterpressure drum; the reflected points are generally related to registration lines, to markings, or to a proof provided on said sheet of paper.
The precision required in aligning the printing plates of the plate-holder cylinders and the markings on the counterpressure drum for quality multiple-color prints must not be lost when, after inking the printing plate, said beam and the cylinder to which the printing plate is fixed are raised, for proofing, into the corresponding position, in which the plate-holder cylinder comes into contact with the counterpressure drum; there is an appropriate kinematic linkage between the cylinder and the drum, which also acts as an intermediate element when it is necessary to turn said cylinder and said drum for said proofing so that they have the same peripheral speed.
When dealing with rubber printing plates and with their considerable deformability, the results obtainable with fixing performed by means of an optical apparatus are currently still satisfactory, despite the expenditure of time required.
Nevertheless, the difficulties involved in correcting observation and machine errors (with the need to adjust the angle of the semitransparent mirror and to correct parallax errors) and in handling large printing plates must be taken into account.
Optical plate mounting devices are less appreciated when dealing with inextensible printing plates (photopolymeric ones or rubber ones provided with an antistretch layer); their very feature of being substantially inextensible is not used fully, especially in terms of quickness in mounting.
Mounting becomes quicker if the inextensible printing plates have been precisely provided beforehand with registration holes and if the plate mounting device has been provided with an adapted bar or surface. Said bar or surface in practice occupies the entire width of the plate mounting device, as far as they are parallel to the counterpressure drum and their corresponding dimension is substantially equal to the length of the drum; they assume an active position and a spaced position and are provided with adjustable pins for engaging said holes of the printing plates. In known plate mounting devices, for example, the pin supporting bar is provided frontally and is fixed to the optical apparatus.
Rather expensive plate mounting devices have more recently become available only for inextensible printing plates provided with microdots or with register marks, said plate mounting devices being no longer provided with an optical apparatus equipped with a semitransparent mirror but with video cameras and respective monitors forming systems that greatly magnify the registration markings (for example 140 times).
The monitors are fixed, whereas the video cameras are positionable along a bar arranged in front of the plate mounting device and lying above the plate-holder cylinder. Before fixing the printing plates onto the plate-holder cylinders, said microdots or register marks, seen by the video cameras on the plate-holder cylinder, are aligned with electronically generated register marks shown on the monitors.
In some cases the position of the video cameras can be controlled by a computer; accordingly, the video camera can easily resume given positions only after demanding computer programming.
In other cases, the video cameras must be positioned manually, and this requires great accuracy; essentially, a given job requires a number of video camera pairs equal to the number of printing plates that must be fixed to each plate-holder cylinder during that print job; accordingly, the number of printing plates that can be fixed to a plate-holder cylinder is rather limited, since the number of pairs of video cameras that can be applied to a plate mounting device is limited.